Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode while also serving as an electrical insulator.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel, and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Each electrode has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode, and reduction of oxygen at the cathode. Protons flow from the anode through the ion conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell. Typically, the ion conductive polymer membrane includes a perfluorosulfonic acid (PFSA) ionomer.
The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells in stacks in order to provide high levels of electrical power.
Polymer electrolytes play an important part in determining the efficiency of PEM fuel cells. To achieve optimal performance, the polymer electrolyte must maintain a high ionic conductivity and mechanical stability at both high and low relative humidity. The polymer electrolyte also needs to have excellent chemical stability for long product life and robustness. Polymeric electrolytes having perfluorosulfonic acid groups are under active development for fuel cell applications.
At present, the coupling of perfluorosulfonic acid groups to polymeric backbones is limited to halogenated aromatic polymers, i.e., those with phenyl-Br or —I moieties and is restricted to coupling with I—CF2CF2OCF2CF2SO3−K+ using metallic copper. The use of organocuprate reagents expands the scope of the coupling reactions to include more aliphatic containing polymers. Presently, halogenated-aromatic polymers are allowed to react with metallic copper and then are allowed to react with I—CF2CF2OCF2CF2SO3−K+ to form polyolefins with perfluorosulfonic acid side groups. An alternative route involves the addition of metallic copper to I—CF2CF2OCF2CF2SO3−K+ in solution, followed by the addition of the halogenated-polymer in solution. The other alternatives are expensive commercially available PFSAs.
Accordingly, an improved method of making polymeric electrolytes is needed, and in particular, to methods of making such electrolytes with perfluorosulfonic acid side groups.